This invention relates to optical imaging systems employing a series of reflecting elements and, more particularly, to a system having multiple reflections from sets of zones on each of a set of mirrors for exclusion of light from sources outside the viewing angle of the system.
Optical imaging systems are employed over various portions of the spectrum including both the visual and the infra-red portions of the spectrum. Our particular interest is the imaging of objects at long distance by telescopes and cameras. Optical elements including both lenses and mirrors may be employed in such instruments, the mirrors being advantageous in regions of the spectrum in which totally transparent lens material may not be available or when multiple wavelength capability is desired. The mirrors are also preferred for their rigidity to insure dimensional stability in the case of very large elements, and in situations wherein a large amount of curvature is required in an optical surface.
One problem attendant the use of telescopes and cameras becomes apparent in the viewing of an object which radiates relatively weak light in a situation wherein there is a relatively strong source of light nearby, in terms of angular spacing. Light from the strong source may enter the optical system and produce a glare which masks the image of the weakly radiating object. A narrow viewing angle lessens the amount of light received and/or scattered from the strong source.
A narrow viewing angle can be obtained in a telescope or camera constructed with mirrors by reflecting the rays of light back and forth between a pair of mirrors, such as between a primary mirror and a secondary mirror which are curved to have a focussing power. For example, the primary mirror may be concave and the secondary mirror may be convex. The multiple reflection permits reduction in the physical size of the telescope to provide the advantage of a more compact telescope. One such optical system having multiple reflections is shown in FIG. 1 of U.S. Pat. No. 3,527,526 issued in the name of Silvertooth or in copending patent application, Ser. No. 06/636,892 filed Aug. 2, 1984 by John P. Bagby, one of the inventors herein. However, the problem remains in that such optical systems may not provide as much isolation of the radiation from the strong source as may be desired. In addition, a further problem arises in that the conventional continuous optical surfaces utilized in the multiple reflections, such as spherical, paraboloidal, aspheric and ellipsoidal surfaces, introduce a compromise between the attainment of even the narrow viewing angle desired versus the desired clarity in the image of the object being viewed, even if only slightly off the optical axis.